I guess the fact that grape names are not translated into English (and not italicised either to indicate a foreign word) is an indication that they are proper nouns.
Sure, some countries and regions have their own names for grapes, which very occasionally amount to a translation, e.g. Pinot Grigio and Pinot Gris, but you never hear of "black pinot", "grey pinot" or "white muscat with little grapes".